The present invention relates to high temperature pressure transducers and more particularly to a hermetically sealed high temperature pressure transducer assembly and a method for making the same.
In recent years the need for semiconductor pressure transducers that can be used in applications that require operation in harsh environments that are corrosive, oxidizing, high vibration and involve high temperatures has increased. Accordingly, not only must the stress-sensing network of these transducers be protected from these harsh environmental conditions in some way to enable the transducer to remain operational at high temperature over extended periods of time, but the entire transducer structure, including: electrical contacts, lead-outs, interconnects and external wiring must also be protected. A method for fabricating a sensor network which is dialectically isolated from the flexing sensor diaphragm can be seen in commonly assigned U.S. Pat. No. 5,286,671, entitled xe2x80x9cDiffusion Enhanced Fusion Bondingxe2x80x9d, the entire disclosure of which is incorporated by reference herein. Therein, the degenerate P+ sensor network remains electrically isolated from the deflecting diaphragm. A method for fabricating such a dialectically isolated sensor structure wherein only the back side of the transducer is exposed to ambient pressure while also hermetically sealing the front side of the transducer which contains the stress-sensing network to a cover member can be seen in commonly assigned, copending U.S. patent application Ser. No. 09/160,976, the entire disclosure of which is also incorporated by reference herein.
Therein, a semiconductor sensor device comprising a semiconductor diaphragm member having a front surface coated with an oxide layer; P+ sensor elements fusion bonded to the oxide layer at a relatively central area of the diaphragm; P+ finger elements fusion bonded to the oxide layer extending from the sensors to an outer contact location of the diaphragm for each finger; and a rim of P+ material fusion bonded to the oxide layer and surrounding the sensors and the fingers is disclosed. A first glass wafer member is electro-statically bonded at the front surface to the fingers and rim to hermetically seal the sensors and fingers of the diaphragm member. The first glass wafer includes a depression above the sensors and a plurality of apertures, where each aperture is associated with a separate finger at the contact location and is smaller than the associated finger lining up with the contact location. Further, each contact location can be accessed via the associated aperture in the first glass wafer.
The apertures in the first glass wafer are filled with a glass-metal frit such as gold or platinum palladium silver, and a second glass wafer or header is sealingly coupled to a top surface of the first glass wafer. The second glass wafer or header has a plurality of apertures aligned with the plurality of apertures in the first wafer and contains a group of hermetically sealed pins slightly protruding from its surface for electrically coupling by means of the glass frit to the various contact locations. In this way it is possible to produce a sensor assembly wherein only the front non-active side of the structure needs to be exposed to the pressure medium, where there is no need for small ball bonded gold leads to the sensor network, and where the entire sensor network and contact area is hermetically sealed and thus not exposed to the pressure media. Such a sensor is illustrated in FIG. 1.
However, the hermetically sealed sensor bonded to a header is only the starting point for an entire transducer assembly. For instance, most transducers must be affixed to a mounting surface for exposure to the pressure media, frequently by means of a threaded port. Thus, the sensor-header assembly must be joined to the port. Additionally, the header pins must be electrically connected to a high temperature cable assembly without the use of solder joints, which could fail at these high temperatures. The high temperature cable assembly must also contain material which will provide electrical isolation between individual leads, while the interconnects between the header and the cable as well as the cable itself must be strong enough to withstand mechanical stress imparted thereon. It is an object of the present invention to provide a structure that overcomes these problems.
A high temperature pressure transducer suitable for mounting on a jet engine or airframe may be made by the following steps:
A reverse mountable absolute Silicon-on-Oxide sensor is fabricated using the processes described in U.S. Pat. Ser. No. 09/041,228.
The apertures in the glass structures are filled with a glass-metal frit and the sensor is mounted with a pyroceram type glass to a header containing small protruding pins of about 0.010xe2x80x3 in diameter which align with the apertures In the sensor structure. The sensor header assembly is then heated causing the various glasses to solidify.
A sleeve is welded to the first header. The second header is used containing a group of tubes which align with the pins of the first header, with the inner diameter of the tube big enough to accept the pin from the first header as well as another lead which will enter the tube from the other direction.
The pins from the first header are inserted through the tubes of the second header and the second header is welded to the sleeve.
A mineral insulated cable containing nickel wires of diameter about 0.020xe2x80x3 is used to interconnect to the pins from the first header. The cable consists of a steel tube of 0.093xe2x80x3 in diameter filled with Mg O which acts as a high temperature insulator and serves to insulate the individual wires from each other and the outer steel tube.
Both the ends of the mineral insulated cable are first sealed with glass but the ends of leads from the cable are left protruding The exposed leads from one end of the cable are inserted into the tubes of the second header. The tubes are then crimped and welded to insure that the header leads (pins) maintain electrical contact with the leads in the mineral insulated cable.
The header-insulated cable assembly is then inserted into a port and welded to the port. At the end of the port there is another tubulation which is also crimped to keep the mineral insulated cable in place. Typically, the mineral insulated cable will be 2 feet in length. This was the finishing stage for most devices using high temperature mineral insulated cable but it is clear that there are certain disadvantages to the transducer in this state. For instance, the interconnect portion between the leads of the first header and the leads of the mineral insulated cable are exposed to a high temperature oxidizing atmosphere. In addition, the only means of securing the cable to the header are the crimps between the header wires and the cable wires and the crimp on the cable at the pressure port. This method of construction leads to a number of unanticipated advantages. Although, the cover sleeve was added to strengthen the mineral insulated cable, the weld of the sleeve to the back of the port and the use of a third header makes all of the internal interconnects hermetically sealed from any atmospheric contamination or oxidation. Every single internal metalized surface such as metal-to-silicon, metal-glass frit, header pins to header tubes, header pins to mineral insulated cable wires and even the mineral insulation itself is hermetically seated from the atmosphere. In addition the welding of the sleeve to the port together with the addition of the third header greatly increases the structural integrity of the entire electrical interconnect system and reduces the chances of any damage in severe environments